Influence of fly ash on the properties of magnesium oxysulfate cement

Fly ash is the main solid waste produced from coal-fired power plants, which has been extensively used as a mineral admixture for concrete and cement. In this paper, the effects of fly ash on the compressive strength, water resistance, setting time, phase composition and microstructure of magnesium oxysulfate (MOS) cement modified with 0.5% sodium dihydrogen phosphate (NaH₂PO₄) were investigated. X-ray diffraction and the Rietveld method were used to analyze the phase composition. Scanning electron microscopy and mercury intrusion porosimetry were conducted to characterize the microstructure. The results reveal that the addition of fly ash can decrease the compressive strength but improve the water resistance of MOS cement. Due to the dilution effect of fly ash, the setting time of MOS cement increases with the increase in the amount of fly ash. Although the filling effect or pozzolanic effect of fly ash brings an improvement in the microstructure of MOS cement, the addition of fly ash reduces the formation amount of 5Mg(OH)₂·MgSO₄·7H₂O, which is an important factor affecting the strength development of MOS cement.